Combustion engines such as diesel engines, gasoline engines, and gaseous fuel-powered engines are supplied with a mixture of air and fuel for combustion within the engine that generates a mechanical power output. In order to maximize the power output generated by this combustion process, the engine is often equipped with a divided exhaust manifold in fluid communication with a turbocharged air induction system.
The divided exhaust manifold increases engine power by helping to preserve exhaust pulse energy generated by the engine's combustion chambers. Preserving the exhaust pulse energy improves turbocharger operation, which results in a more efficient use of fuel. In addition, the turbocharged air induction system increases engine power by forcing more air into the combustion chambers than would otherwise be possible. This increased amount of air allows for enhanced fueling that further increases the power output generated by the engine.
In addition to the goal of maximizing engine power output and efficiency, it is desirable to simultaneously minimize exhaust emissions. That is, combustion engines exhaust a complex mixture of air pollutants as byproducts of the combustion process. And, due to increased attention on the environment, exhaust emission standards have become more stringent. The amount of pollutants emitted to the atmosphere from an engine can be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of exhaust emissions includes utilizing an exhaust gas recirculating (EGR) system. EGR systems operate by recirculating a portion of the exhaust produced by the engine back to the intake of the engine to mix with fresh combustion air. The resulting mixture has a lower combustion temperature and, subsequently, produces a reduced amount of regulated pollutants.
EGR systems require a certain level of backpressure from the exhaust system to push a desired amount of exhaust back to the intake of the engine. And, the backpressure needed for adequate operation of the EGR system varies with engine load. Although effective, utilizing exhaust backpressure to drive EGR can adversely affect turbocharger operation, thereby reducing the air compressing capability of the air induction system. The reduced air compressing capability may, in turn, reduce the engine's fuel economy and possibly the amount of power generated by the engine. Thus, a system is required that provides sufficient and variable exhaust backpressure to drive EGR flow without adversely affecting turbocharger or engine operation.
An example of a turbocharged engine have exhaust gas recirculation is disclosed in U.S. Pat. No. 6,694,736 (the '736 patent) issued to Pflüger on Feb. 24, 2004. In particular, the '736 patent discloses an engine with a common intake manifold and divided exhaust manifolds. Two high-pressure turbochargers having respective high-pressure compressors connected to and driven by high-pressure turbines are separately associated with the common intake manifold and the two exhaust manifolds, and a single low-pressure turbocharger receives exhaust from each of the two high-pressure turbochargers (i.e., the engine of the '736 patent includes three turbochargers arranged into two stages). In addition, exhaust return pipes are connected to the intake manifold downstream of the high-pressure compressors to direct exhaust from upstream of the high-pressure turbines back into the engine.
The disclosed exhaust system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.